mail.o7.com has no CNAME records.
Question and response
Question
dig mail.o7.com. NS
Answer
The response has no answer records.
Authority
o7.com. 160 SOA ns5.o7.com. support.cloud.rt.ru. 2016051535 600 10800 2419200 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0